Isolated converters are widely used in situations with high safety requirements and off-line power supply. A control is desired with high accuracy, high efficiency, low EMI, small size and low cost. Feedback control is essential to accurately adjust the energy delivered to the load. For isolated converters, a basic requirement of feedback is electrical isolation between the primary side and secondary side.
An optical coupler is commonly used in feedback control to achieve electrical isolation. The output voltage information at the secondary side is fed back to the primary side through the optical coupler to accurately control the primary side switch delivering the optimal energy to the secondary side. One drawback of employing an optical coupler is that it increases the system cost. Further, an optical coupler can be damaged under high isolation voltages. For example, in medical instrumentation applications, power supply systems require high reliability and often suffer from high voltage spikes.
Another isolated feedback approach uses a dedicated third winding or auxiliary winding. The output voltage at the secondary side is similar to that in the auxiliary winding. Thus, by detecting the voltage at the auxiliary winding side, feedback information can be obtained. However, there exist problems while adopting the feedback approach. One problem is that it can not accurately reflect the output level, especially during a load transient. Like the optical coupler, the auxiliary winding approach increases cost. Thus, it is a challenge to realize the voltage feedback control accurately in a simple way.
Besides cost and accuracy considerations, high efficiency and low EMI are also desired. One approach is using soft switching technology such as ZVS (zero voltage switching) to reduce the switching loss. For ZVS, the drain-source voltage of the switch is zero when the switch is turned on so that there is no turn-on loss. Also, a snubber capacitor can be paralleled directly with the power MOSFET. The dv/dt is greatly reduced, which not only lowers the turn-off loss, but also reduces EMI significantly. With the reduction of the switching loss, the converter can run at higher switching frequency, which reduces the sizes of transformer and other passive components.
The quasi-resonant converter is a typical isolated converter with soft switching and feedback control. FIG. 1 shows a prior art topology of a quasi-resonant converter wherein Lp is the inductance of the primary winding, Rp is the resistance of the primary winding, Cp is the resonant capacitor, and Ld is the inductance of the auxiliary winding. When the energy at the secondary side depletes (magnetic flux resetting), there is an oscillating voltage at the drain of the primary switch Qp. The resonant frequency is determined by Lp and Cp, and the attenuation factor is decided by Rp. An auxiliary winding Ld is employed to detect the magnetic flux resetting and thus control Qp turn-on at the bottom of the oscillating voltage to decrease the switching loss. Meanwhile, an optical coupler is used to feedback the output voltage information to the primary side to regulate the energy delivered to the secondary side. As described above, the quasi-resonant converter adopts both the auxiliary winding and an optical coupler to realize feedback and soft switching. However, it can not ensure a zero voltage switching and it has large size and high cost.